http://www.gnu.org The spiritual home of the free-software OS movement. Learn much about command-line tools here.
http://www.linuxdevcenter.com/linux/cmd The alphabetical directory of Linux commands. Information on 515 (and counting) Linux commands.
http://tuxfiles.org/linuxhelp/cli.html Newbie-oriented help on assorted command-line topics.
http://linuxcommand.org You have Linux installed and running. The GUI is working fine, but you are getting tired of changing your desktop themes. You keep seeing this "terminal" thing. Don't worry, we'll show you what to do.
http://justlinux.com/nhf/Shells/Basic_Console_Commands.html More on the commands listed here.